An exemplary system for producing electrical energy from heat energy consists of the following main components: a feed pump for conveying liquid working medium to an evaporator by increasing the pressure, the evaporator itself for evaporating and optionally additionally superheating the working medium by supplying heat, an expansion machine, in which the high-pressure evaporated working medium is expanded and thereby produces mechanical energy, which, for example, can be converted into electrical energy by means of a generator, and a condenser, in which the low-pressure steam (expanded working medium) from the expansion machine is subcooled and condensed. The condensed working medium returns from the condenser to the feed pump, whereby the thermodynamic cycle is closed. In the case that the working medium is an organic working medium, the thermodynamic cycle is an Organic Rankine Cycle (ORC system).
In order to avoid cavitation in the feed pump, the condensed working medium is subcooled, thus, cooled to a temperature, which is below the condensation temperature (equivalent to the boiling temperature) at the condensation pressure. In this way, the NPSH value (Net Positive Suction Head) is achieved
There are basically two possibilities to implement the condenser of a thermodynamic cycle (in particular an ORC system). On the one hand, the condensation of the working medium can be liquid-operated (e.g. water-cooled) or the condensation, on the other hand, can be air-operated. Water-cooled condensation offers the advantage that the condensation heat can be fed into a heating circuit and, thus, is available to the heat consumers (e.g. a stable, a building heating system, a fermenter, etc.). If there are no heat consumers, only an air-cooled condensation is possible, however, thereby, the own requirements of a fan are at the expense of the electrical efficiency.
There are also applications for which heat consuming is desired only for a limited time of the year. If, however, the heat use and the electricity production are to be made possible by the ORC, the surplus heat has to be emitted, for example, via the emergency cooler of a combined heat and power station in the time of the year, in which a heat consumption does not take place. However, this is associated with a high power consumption and, thus, with increased costs.
Basically (according to an internal non-published prior art of the applicant), two condensers can be interconnected in order to allow both operation modes (air-cooling and liquid-cooling, in particular water-cooling). However, the difficulties here are to regulate the distribution of the mass flows of the working medium in the respective condensers and, thus, the heat emission. The aim is to enable a heat quantity, which is as large and defined as possible in a condenser integrated in a heating circuit.
In order to regulate the mass flows, mechanical valves, as, for example shut-off valves, can be used. However, this involves the problem that different pressure levels are present in both condensers. This may lead to the return flow of the condensed fluid into the condenser with the lower pressure until this condenser is completely filled up. However, by the valves to be installed, the complexity of the system as well as the error rate is increased, as the correct valve positions have to be kept for the correct operation modes.